Not applicable.
Not applicable.
The invention generally relates to oscillometeric blood pressure determining techniques, and more particularly to determining the diastolic pressure using that technique.
Knowing the pressures exerted by blood on the artery walls of patients is of great value to those engaged in medical practice. In the case of humans, the pressure in the vascular system is measured for many reasons, including diagnosis, ascertainment of the progress of therapy, the physiological state when under anesthesia, etc. As an example, the determination of arterial blood pressure is an essential element in the diagnosis of a patient suspected of cardiac disease. Normal human arterial blood pressure ranges between 80-120 millimeters of mercury, whereas elevations of arterial blood pressure above that range are found in cases of congestive heart failure, renal artery disease, coarctation of the aorta, etc. Additionally, untreated hypertension is known to be associated with an increased risk of stroke, coronary artery disease, and aneurysms.
During the cycle of the heartbeat the arterial blood pressure oscillates. When the heart muscle contracts, known as systole, blood is pushed into the arteries. This increases the arterial pressure. When the heart muscle relaxes, known as diastole, the arterial blood pressure falls. The maximum of the arterial pressure oscillation during the heartbeat is known as systolic pressure; the minimum is known as diastolic pressure. The arterial pressure versus time waveform can also be used to calculate what is known as mean arterial pressure. The mean arterial pressure (MAP) is calculated by integrating the arterial pressure waveform for one cycle and then dividing that quantity by the cycle period. The indirect techniques of oscillometry and auscultation are used in practice to estimate the systolic, mean, and diastolic pressures non-invasively. However, it is known that under certain conditions the diastolic estimate that oscillometry produces is inaccurate, yet the systolic and MAP estimates are good. It is the purpose of this invention to improve the diastolic estimate using easily obtained, but previously ignored oscillometeric information.
The auscultatory method is commonly use by medical personnel to indirectly measure arterial blood pressure. In this technique, constrictive pressure is gradually applied about the limb of the patient until the flow of blood through the limb vessel has been arrested, as determined by listening to a stethoscope applied over the vessel at a point distal the point of constriction. Then upon gradual release of the constriction pressure, the beginning of the flow through the vessel can be heard and the constriction pressure is noted on a gauge reading in millimeters of mercury. This pressure is referred to as systolic pressure and is taken as an estimate of the true intra-arterial systolic pressure. The pressure then is gradually released further until the sounds of the flow again cease and the pressure is again noted, which pressure is referred to as diastolic pressure and is taken as an estimate of the true intra-arterial diastolic pressure. Previously the constriction pressure has been derived from an inflatable cuff connected to a mercury column manometer or to an aneroid type gauge having a dial scale calibrated in millimeters of mercury. It is also known that the auscultatory estimate of diastolic pressure can at times be inaccurate; auscultation can be very technique dependent and varies, for example, due to the hearing ability of the clinician taking the reading. Furthermore, auscultation can, in some cases, be quite confusing when determining diastolic estimates because the Korotkoff sounds may never disappear as the cuff pressure is lowered.
A previous automatic indirect blood pressure reading apparatus employed the oscillometeric method in which an arm cuff is inflated to a pressure at which blood flow is occluded. The cuff then is deflated at predetermined pressure increments in a step-wise manner. At each step, the pressure in the cuff is measured repeatedly using a suitably short sampling period in order to detect pressure fluctuations. The instantaneous pressure in the cuff is due to the inflation pressure and the force exerted by the pressure pulsations in the patient""s blood artery during each heartbeat. The beating heart can cause the pressure in the cuff to oscillate at a deflation step. The apparatus continues in this fashion until a complete envelope of oscillation amplitude versus cuff pressure is obtained. The cuff pressure at which the maximum amplitude oscillations are obtained is indicative of the mean arterial pressure. The systolic and diastolic pressure estimates are also determined from predefined functions of the envelope data. The oscillometrically determined systolic, MAP, and diastolic are considered estimates of the true intra-arterial pressure values. However, it is also known that arterial compliance plays a major role in the estimating functions; arterial compliance can change in complicated and unpredictable ways as physiological circumstances change.
The oscillometric blood pressure is determined indirectly from a cuff that is placed around a portion of the body, such as an upper arm, of the subject whose blood pressure is desired. The cuff is inflated to a predetermined pressure, preferably great enough to occlude the flow of blood in the limb of the patient. Then the cuff is deflated in a controlled manner to produce a deflation pressure in the cuff that decreases with time. In the preferred embodiment, the cuff is deflated in regular pressure increments thereby producing a plurality of discrete deflation pressure levels.
During each of a plurality of heartbeats, the pressure oscillations that occur at the discrete deflation pressure levels are measured and stored in the apparatus. The complete data set of the amplitude of the oscillations versus the discrete pressure levels is known as the oscillometric envelope. The oscillometric estimate of the mean arterial pressure is determined from this envelope data. For example, the estimate of the mean arterial pressure is the deflation pressure level that occurs when the oscillation measurements have the greatest amplitude. Similarly, the systolic pressure can be estimated from the envelope data by finding the discrete deflation pressure level which occurred when the oscillation amplitude is a predetermined fraction of the maximum oscillation size.
In the preferred embodiment, the waveform of the oscillation pressure is acquired from the cuff during a single cardiac cycle at a deflation pressure level that is expected to be less than the diastolic pressure of the subject. The cuff oscillation waveform shape is known to reflect the intra-arterial waveform oscillation best at a cuff pressure level that is slightly less than the diastolic pressure, whether it is derived from a filtered or unfiltered cuff pressure signal. However, its DC level is obscured by the oscillometric technique. The diastolic pressure can be initially estimated by the oscillometric technique. This first diastolic estimate can be used to select the level from which the oscillation waveform used for calibration comes. The mean value for the oscillation pressure waveform at the sub-diastolic discrete deflation pressure level is calculated. The process then associates the mean value of this oscillation pressure waveform and the estimate of the mean arterial pressure as determined from the oscillometric envelope. Similarly, the peak of the oscillation pressure waveform at the sub-diastolic discrete deflation pressure level is associated with the systolic pressure as estimated from the oscillometric envelope. In this way the oscillation waveform at the sub-diastolic discrete deflation pressure level becomes calibrated, i.e. each sample value of the waveform corresponds to a particular pressure.
Mathematically, as one example, the calculation can involve finding differences based on the blood pressure estimates and the oscillation waveform data obtained. A first difference is derived as the difference between the oscillometric systolic pressure estimate and oscillometric mean arterial pressure estimate and is called Delta1. A second difference is produced by subtracting the oscillation waveform mean from the oscillation waveform peak and is called Delta2. The process also determines a third difference by subtracting the oscillation waveform minimum from the oscillation waveform peak and is called Delta3.
The exemplary process culminates by determining the diastolic pressure from: the oscillometric systolic pressure estimate, Delta1, Delta2, and Delta3. This is accomplished in the preferred embodiment by deriving a quantity called the scaled difference by multiplying Delta1 by Delta3 and dividing by Delta2. Subtracting the scaled difference from the oscillometric systolic estimate produces the improved value for the diastolic pressure as given by:
Diastolic Estimate=Systolic Estimatexe2x88x92(Delta1*Delta3)/Delta2.
Other formulae can be used within the scope of the present invention.